Innovative Blades May Have Led to a Stone Age Population Boom

Technological innovations have enabled human cultures to thrive, and now researchers have discovered what might be the oldest example known so far of such an occurrence. These ancient innovations are in the form of miniature stone blades, which appear to have contributed to a population boom in south Asia.

Recent genetic research of people across the globe suggests that roughly 45,000 to 20,000 years ago, one of the most dramatic population booms after humanity dispersed from Africa occurred in southern Asia, leading to "the highest population densities in the world in prehistory," explains Michael Petraglia, an archaeologist at the University of Oxford in England.

After studying mitochondrial data from people in India and neighboring regions, Petraglia and archaeologist Ravi Korisettar of Karnatak University in Dharwad, India, and their collaborators refined the timing of this population boom to between 35,000 and 28,000 years ago. "Why this population expansion happened is a bit of a mystery," Petraglia says.

To investigate both the potential causes and effects of this population boom, Petraglia and his colleagues scoured existing archaeological evidence in south Asia. They found that whereas 153 sites of human occupation were found dating back to the middle Paleolithic, or roughly 300,000 years ago, past studies had uncovered some 400 sites dating back to the late Paleolithic, or about 40,000 to 10,000 years ago. The researchers suggest the greater number of late Paleolithic sites support the genetic suggestions of a population boom.

After delving into a site at Jwalapuram in southern India, which has preserved artifacts spanning the past 78,000 years, the scientists also discovered signs of technological innovation at roughly the same time as the population boom. Until 38,000 years ago, blades made of stone flakes were squat or relatively large. But afterward a new kind of blade came to the forefront—small, elongated "microliths" just four centimeters in length or less, with triangular, crescent or trapezoidal shapes. Similar findings were seen during the population boom at the Patne site on the west coast of India and at the Fa Hien Cave site in Sri Lanka.

Microliths, which might have been parts of barbed weapons, "were a key innovation," Petraglia explains, which until now were thought to have emerged in southern Asia only 10,000 years ago. "The fact that they are light, portable, versatile and straightforward to replace is quite an improvement over prior technologies—you can make hundreds of them quickly and repair your spear or arrow very quickly, making them easier to maintain. In earlier periods, if you lost or broke your stone tool, you would have to invest a lot of time and effort in remanufacturing one."

The advance in technology might have helped burgeoning human populations make the most out of the environment, Petraglia speculates. The researchers also found that while populations were expanding and technology was improving, the environment in south Asia was deteriorating—climate shifts cut down on monsoon rainfall, leading to growing deserts with pockets of rich, diverse resources. Technological innovations could have helped catch more food in those tough times, allowing people to thrive, despite the environmental challenge.

Petraglia notes, however, that cause and effect is extraordinarily hard to determine here, if not impossible. For instance, the environmental downturn could have forced humans to live closer together to make use of resource clusters. As populations grew, more complex technologies were likely to persist, suggests prehistoric archaeologist Lawrence Straus at the University of New Mexico in Albuquerque. "You could have had Leonardo da Vinci 50,000 years ago inventing technology, but if there aren't enough people around to carry on the ideas, the invention would die with him," he explains.

These new findings, published this week online by the Proceedings of the National Academies of Science, are an important key to the puzzle of how technology emerged as humans dispersed across the globe, says archaeologist Ofer Bar-Yosef at Harvard University, who, like Straus, did not participate in this study.

"We see microlith blades in the Near East going all the way to central Asia and Siberia at exactly the same time, but not in Southeast Asia and south China and New Guinea and Australia, so something changed after they reached India, and we're struggling to understand why," he explains. "We now see them in India at this time, which back then was dry, with similar landscapes to other places such as in Africa or the Near East, where we see microliths then. But in Southeast Asia and New Guinea, you're back in tropical forests, so it's possible that we don't see microliths there because of an adaptation to the environment or due to lack of suitable raw materials for making them."